In any conventional refrigeration system, the evaporator or air unit is subject to frosting over when the evaporator temperature is below freezing point. One method of preventing the evaporator from clogging up with frost is to apply an external heat source, such as an electrical heater or water, to the exterior of the air unit which warms it up enough to melt the frost. Another method is to release hot, gaseous refrigerant stored in the receiver of the refrigeration system to the interior of the air unit pipes. Using hot refrigerant from the refrigeration system is preferable to an external heat source, because the hot gas is a by-product of a refrigeration system and an additional energy source is not required.
There are also times when it is desired to warm the enclosure such as when outdoor ambient temperatures are very low. An external heat source, such as an electric heater can be used, but again it is desirable to heat the enclosure using the hot, gaseous refrigerant from the receiver as above. This hot, gaseous refrigerant is routed through hot gas piping either to the air unit in a heating mode or to a separate re-heating coil for heating the enclosure or to control its relative humidity.
However, as the temperature of ambient conditions surrounding a condenser of a refrigeration system sized for the summer conditions begins to drop, the condenser is too large and becomes able to condense the refrigerant at much lower pressures. The pressure of the refrigerant leaving the compressor decreases, and the receiver fills substantially with liquid refrigerant at a lower pressure. In this situation, there is less hot gas refrigerant available, and because the condenser pressure is lower, the available hot gaseous refrigerant is also at a lower pressure and temperature. Many times there is not enough pressure and the gaseous refrigerant is not hot enough to provide ample defrosting to the air unit, to provide heating to the enclosure, or to operate the re-heat coil.
Therefore, there is a need for a refrigeration system that can produce enough hot, gaseous refrigerant at a controlled pressure to defrost air units, operate air units in a heating mode, or operate the re-heat coils, when ambient conditions around the condenser are low.